Reaction force transmission mechanism for a pneumatic brake booster

ABSTRACT

The present invention relates to a reaction force transmission mechanism for a pneumatic brake booster including a largely incompressible reaction plate that is interposed between an input member and an output member and encased at the peripheral side and has lateral abutment surfaces for the input member and the output member, wherein the abutment surface close to the input member can be acted upon by a pressure member which is adapted to be fixed to a control housing and, consequently, can be excepted from the transmission of the reaction force to a vehicle driver. To improve its service life, the reaction plate includes a recess in the area of the abutment of a valve piston extension, in which recess a substantially incompressible, elastic insert is placed.

TECHNICAL FIELD

The present invention relates to a reaction force transmission mechanismfor a pneumatic brake booster.

BACKGROUND OF THE INVENTION

Reaction force transmission mechanisms principally comprise reactionplates which, at least at their periphery, are received in a controlhousing and act under pressure like an incompressible fluid. Morespecifically, the reaction plate establishes the balance of forcesbetween the foot pressures exerted by the vehicle driver on the brakesystem and the reaction forces on the part of the brake system.Consequently, the pressure forces applied to the plate are convertedinto a change in shape with the limiting conditions of a constant volumeand a constant pressure in the interior of the plate. A reaction plateof this type is e.g. disclosed in DE 11 92 532 A1 and basicallysatisfies the demands placed on conventional apparatus.

Recently, however, there is a trend to using brake boosters withtransmission mechanisms which provide a panic stop function. These brakeboosters are characterized by the fact that a normal brake applicationor an emergency brake operation with full power boosting is initiated independence on a predetermined relative displacement between an inputmember (valve piston) and a control housing that receives the reactionplate at least in part, which necessitates only low (foot) brake forcesin comparison to priorb-art pneumatic-mechanic brake boosters. A brakebooster of this type is e.g. disclosed in EP 901 950 A1 and comprises apressure member which can be fixed to a control housing by way ofcoupling means. The comfort of actuation is, however, considered worthyof improvement because no counterforces or only low counterforces, whichare not in conformity with the normal reaction force of the brakesystem, are transmitted to the vehicle driver in the emergency brakeposition.

From WO99/26826 another brake booster with a panic stop function isknown which, as far as the reaction of forces is concerned, offerscertain advantages, but nevertheless requires improvements with respectto the useful life of the reaction plate because the front end of thevalve piston acts with a comparatively small effective surface on thereaction plate, which causes increased stress.

BRIEF SUMMARY OF THE INVENTION

It has been found that the problems are overcome in that the reactionplate includes a recess in the area of the abutment of the valvepiston's extension, and in that a substantially incompressible, elasticinsert is placed into the recess. This measure permits reducing stress,in particular shearing strains, in the inside of the reaction plate and,consequently, increases the service life of the reaction plate.

It is advantageous in another embodiment of the present invention thatthe valve piston's extension is configured as a separate tracer pin, andthe diameter of the tracer pin in the area of its abutment on thereaction plate corresponds to the diameter of the recess. Thus, theradial outside part and the insert are separated in the area of the mostintense (shearing) strains.

Finally, it is favorable that the insert and the outside part are madeof materials with a different elasticity module (or different Shorehardness). This permits adapting the brake force booster with respect tothe switch-on threshold of the panic stop function.

Another favorable effect is achieved by that, starting from the abutmentsurface of the output member (push rod) on the reaction plate side, anaxial recess is provided into which the insert is additionally slipped.Thus, the circular-cylindrical insert (in the non-deformed condition)has a length greater than the thickness of the reaction plate. Thisimparts a greater volume to the insert and allows the insert totransform pressure stress into deformation work to a still greaterextent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a control unit according to an embodiment in a readyposition of the brake.

FIG. 2 is a view of the control unit according to FIG. 1 in the brake'sfully applied position (panic stop function).

FIG. 3 is a view of the control unit of FIG. 2 in a release position outof the brake's fully applied position.

FIG. 4 is a cross-sectional view as in FIG. 1 of another embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The control unit 1 of a pneumatic brake booster including a reactionforce transmission mechanism with a panic stop function, as shown in theFigure, comprises an input member 2 (valve piston), an output member(push rod) 3, two sealing seats 4, 5, a control housing 6, a reactionplate 7 arranged between input member 2 and output member 3, and furthercoupling means 8 for a pressure member 9, and corresponds in basic partsto the control unit disclosed in FIG. 9 of WO 99/26826 so that thepertinent parts of the description are expressly included in the presentapplication. The activation operation and the release operation for thepanic stop function are identical with the functions of the presentinvention because in each case a relative displacement between inputmember 2 and control housing 6 is taken into account as a criterion forthe triggering operation.

In contrast to the prior-published reaction force transmissionmechanism, the reaction plate 7 includes a recess 13 in the area of anabutment surface 10 of a valve piston extension 11 (according to thisembodiment a tracer pin 12).

The tracer pin 12 is a component which is manufactured independently ofthe input member 2 and mounted in a bore 14 of the pressure member 9 soas to be slidable in relation to input member 2 and also to the pressuremember 9. With a first end 15, tracer pin 12 is movable into abutment ona frontal end 16 of the valve piston, and with a second end 17 havingabutment surface 10 is movable into abutment on the reaction plate 7.With respect to its surface contents, the abutment surface 10 of thetracer pin is smaller than the abutment surface 18 of the pressuremember 9. A (small) portion of the reaction force is transmitted ontothe input member (valve piston) 2 and, hence, to the driver due to theabutment of the tracer pin 12 on the reaction plate 7, on the one hand,and on the input member 2, on the other hand, which abutment takes placein the emergency braking position (FIG. 2). Consequently, the driver notonly senses the reactive effect of spring forces but also of brakereaction forces. More specifically, an actuating feeling is imparted tothe vehicle driver which corresponds to the brake effect. Preferably,the diameter of the tracer pin 12 at the first end close to the reactionplate is identical to the diameter of recess 13. An exchangeable insert19 made of elastic material is disposed in the recess. This measurepermits effectively reducing mechanic strains in the interior of thereaction plate 7 and, consequently, increasing the plate's service life.The separation between the radial outside part of the reaction plate 7and the insert 19 thus takes place in the area of the most intenseshearing strains.

It is feasible to manufacture both the insert 19 and the reaction plate7 from identical material. However, it is advantageous for certain casesof application to make the insert 19 and the outside part 7 of materialshaving a different elasticity module (or different Shore hardness). Thispermits adapting the brake booster with respect to the connectingthreshold or switch-on threshold for the active emergency brakingposition.

In a certain way, the Figures illustrate a cycle of actuations by way ofinstantaneous shots. In a ready position according to FIG. 1, the inputmember 2 is actuated so that the sealing seat 4 is closed (isolation ofthe working chamber from the evacuation). Likewise the control valvewith the sealing seat 5 which is responsible for the ventilation of theworking chamber with atmospheric pressure assumes its closed condition,however, each further displacement of the input member 2 in theactuating direction causes a more or less extensive ventilation. In thiscondition, input member 2, output member 3, and pressure member 9 are inabutment on the virtually pressureless, non-deformed reaction plate 7.The tracer pin 12 is with its first end 15 in abutment on the frontalend 16 of the input member and with its second end 17 in abutment on theinsert 19.

In the activated emergency braking position according to FIG. 2, sealingseat 4 is closed and sealing seat 5 opened, and coupling means 8 takecare of fixing the pressure member 9 to the control housing 6, as can betaken in detail from WO99/26826. Further, there is a balance of theinput-side and the output-side forces at the reaction plate 7. This iseffected by the deformation of the reaction plate 7 shown. It will benoted in addition that the entry of the pressure member 9 and the tracerpin 12 cause a considerable deformation of reaction plate 7 and insert19. Because the insert 19 is compressed between two walls (output member3 and tracer pin 12), it will increase in diameter with a reduced length(under the limiting condition of the constancy of volume).

FIG. 3 illustrates the release process after, starting from the activeemergency braking position according to FIG. 2, the input member 2 hasbeen retracted in opposition to the actuating direction. In doing so,the valve seat 4 was opened for evacuation of the working chamber, andvalve seat 5 closed. Further, it is important that the coupling means 8abut on a transverse member 20 which, during the retraction movement, inturn is movable into fixed abutment on a booster housing (not shown).This causes release of the clutch, as may be gathered in more detailfrom WO99/26826. Due to the retraction movement, on the one hand, anddue to the pressure that is transmitted close to the periphery to theinsert 19 caused by the reaction plate 7, on the other hand, the insert19 flows in a longitudinal direction by reducing its diameter, as shownin FIG. 3. Except for barrel-shaped deformations at the periphery, theinsert 19 largely adopts its original shape.

The present invention may be arranged for in another embodiment of thepresent invention (FIG. 4) that, starting from an abutment surface (21)of the output member (push rod) 3 of the brake booster close to thereaction plate, an axial recess 22 is provided into which the insert 19is additionally slipped. Consequently, the insert 19 which preferably isof a circular-cylindrical configuration has an axial length which isgreater than the thickness of the reaction plate 7 and, in total, has alarger volume than the insert according to FIGS. 1 to 3. A change inshape is thereby allowed to an increased extent.

What is claimed is:
 1. Reaction force transmission mechanism for a pneumatic brake booster, wherein the reaction force transmission mechanism is connected between an input member and an output member, wherein a side of said force transmission mechanism connected to the input member is selectively acted upon by a pressure member which is adapted to be fixed to a control housing and, consequently, can be excepted from the transmission of the reaction force to a vehicle driver, comprising: a reaction plate having a recess, a tracer pin disposed adjacent to an elastic insert, wherein said tracer pin is in operative communication with said input member, wherein the output member includes an axial recess for accommodating the elastic insert, and wherein the elastic insert has an axial length greater than a thickness of the reaction plate.
 2. Reaction force transmission mechanism as claimed in claim 1, wherein the tracer pin is slideably captured by said pressure member, wherein said tracer pin is adapted to bear with a first end against a frontal end of the input member and with a second end against a surface of the elastic insert.
 3. Reaction force transmission mechanism as claimed in claim 2, wherein the pressure member contacts a surface of said elastic insert, and wherein the tracer pin includes an abutment surface in operative contact with the reaction plate, wherein the abutment surface area of the tracer pin is smaller than an abutment surface area between the pressure member and the reaction plate.
 4. Reaction force transmission mechanism as claimed in claim 2, wherein the recess in the reaction plate has a diameter which generally corresponds to a diameter of the tracer pin in the area of its abutment against the reaction plate.
 5. Reaction force transmission mechanism as claimed in claim 1, wherein the elastic insert and the reaction plate are made of materials with different elasticity modules or different Shore hardnesses.
 6. Reaction force transmission mechanism as claimed in claim 1, wherein the elastic insert has a cylindrical shape or a barrel shape.
 7. Reaction force transmission mechanism as claimed in claim 1, wherein the elastic insert is separable from the reaction plate. 